The present invention relates to a method for making improved abrasion resistant overlays for use in making decorative laminates and the like. More specifically, the present invention provides improved an improved method for making abrasion resistant overlays for use in making decorative laminates and the like using grit particles, generally aluminum oxide, that have been micro-encapsulated in a resin, generally melamine-formaldehyde resin.
Decorative laminates are conventionally manufactured by assembling several layers of a sheet material such as paper or fabric impregnated with resins of various kinds. Typically, the resins may be selected from phenolics, aminoplasts, polyesters, polyurethanes, epoxy resins, melamine resins and the like.
The selection of the paper or fabric to be used, and the resin for impregnation is governed by the intended end-use of the finished laminate. For some end uses, surface decoration is not required, but in many instances colors and/or patterns are desired to add eye appeal to the finished laminate. While color and/or pattern decoration may be desired for an outer surface of the laminate, the core or base functions primarily as a strengthening support, and may comprise wood, such as plywood, multiple layers of unbleached or dark colored paper or cloth, and may utilize dark colored, less expensive impregnating resins, such as phenolic resins.
However, when a decorated or printed surface is desired in the laminate, an outer surface layer known as a decor sheet is used to cover the core layer or layers. The decor sheet can be colored decorative paper which may be pigmented with titanium dioxide and/or other opacifying pigments or printed decorative paper, where decorative paper is further printed with patterns to mask the dark-colored core stock. The decor layer may be impregnated with a wide variety of resins such as melamine resins, polyester resin, etc.
Needless to say, given that the decorative laminates discussed herein are often exposed to foot traffic (when used in flooring) or general wear and tear (when used in countertops and the like) it is generally desired to protect the decor layer in some manner that would prevent damage to the decorative image. To impart the desired wear and/or abrasion resistance to these decorated laminates, it has long been the practice to place a resin-impregnated surfacing paper known as an overlay sheet over the decor sheet. Upon consolidating the laminate, the overlay sheet becomes transparent, permitting the printed pattern on the decor sheet to be seen through the overlay sheet. Recently, it has been found that small inorganic particles, known in the industry as xe2x80x9cgrit,xe2x80x9d can be added to the overlay sheet to impart added abrasion resistance to the laminates incorporating them. This grit, which very frequently is comprised of aluminum oxide particles, can also be added directly to the printed decor papers that have been coated with resins. While there have been many methods disclosed for the addition of the grit to these papers, as will be discussed in detail below, there are several methods that are preferred.
However, regardless of the method that is used to incorporate the grit into the paper, be it overlay or decor sheet, the use of the grit in the papermaking process has added heretofore non-existent problems. More specifically, while the addition of grit to these papers has been used effectively to produce laminates having desirable wear-resistant properties, the use of the highly-abrasive grit can cause problems in the papermaking process. For example, the mixing and transportation of the abrasive slurries carrying the grit to the point where the grit is added to the paper itself results in a large amount of wear on the pumps, pipes, and other process equipment used in the process. Additionally, once the grit has been added to the paper, the presence of the grit on the paper significantly adds to the wear and tear on the drying machines and other downstream equipment. Most importantly, though, the presence of the grit in the paper during the lamination process can result in damage and wear to the highly polished caul plates. Thus, for all of these reasons, it is considered highly desirable to lessen or eliminate the wear and tear on the equipment used to make wear resistant overlay paper as well as the caul plates used to laminate that paper while maintaining the wear-resistant properties of the paper itself.
In attempting to address these problems, it has been found that by using grit, such as aluminum oxide particles, which have been micro-encapsulated in a melamine-formaldehyde or a similar type of resin prior to adding the grit to the papermaking process, that wear and tear on the paper and laminating process machinery is reduced while the wear-resistant properties of the end product laminate made using the paper are not significantly diminished. In this regard, U.S. Pat. No. 5,962,134 to Shah et al. discloses one technique for encapsulating grit particles in melamine-formaldehyde resin. However the Shah et al. reference is silent as to preferred methods for incorporating the micro-encapsulated grit into finished paper products. Additionally, AU 9806636 owned by Depco Pty Ltd entitled Wear Resistant Surfaces and Laminates discloses that similar micro-encapsulated abrasive particles can be added to overlay paper during the papermaking process. As with the Shah et al. patent, though, the Depco reference is relatively silent on the incorporation of the micro-encapsulated grit particles into the papermaking process saying only that the micro-encapsulated grit should be incorporated xe2x80x9cwith and or supplementary to the other raw materials.xe2x80x9d Similarly, WO 97/26410 owned by Arjo Wiggins S. A. discloses the utility of the use of micro-encapsulated grit in wear resistant overlay and decor papers for decorative laminates without providing much guidance as to the incorporation of the grit into the papers themselves.
Thus, while these references disclose the general utility of adding micro-encapsulated grit to overlay-type laminate papers, they fail to disclose the preferred methods for incorporating these materials into the papers efficiently and effectively. Accordingly, it would be desirable to have a method for incorporating micro-encapsulated grit particles in overlay or decor laminating papers that is inexpensive, efficient, causes minimal damage to papermaking equipment, and provides finished papers having the desired wear-resistant properties.
The present invention relates to preferred methods for applying micro-encapsulated grit to a fibrous cellulosic overlay or decor sheet, generally paper, in a manner which is inexpensive, efficient, causes minimal damage to papermaking equipment, and provides finished papers having the desired wear-resistant properties. More specifically, the present invention provides methods for producing such paper for use in wear-resistant laminates wherein particles of micro-encapsulated grit are evenly distributed across the surface of the paper and are preferably incorporated in the paper in the z-direction. Additionally, the present invention provides methods for producing such paper for use in wear-resistant laminates that are efficient in the distribution of micro-encapsulated grit on and in the paper fibers, that are efficient in their use of water in the papermaking process, that result in relatively little waste of the micro-encapsulated grit materials, and that can be used in laminates to create the desired wear-resistant and decorative properties. Finally, the present invention provides methods for producing such paper for use in wear-resistant laminates wherein the papermaking equipment and laminating equipment are protected, where possible, from unnecessary wear and tear.
Specifically, the present invention provides methods for producing paper for use in wear-resistant laminates wherein micro-encapsulated grit is deposited on and through the paper by means of the primary headbox or a secondary headbox at the xe2x80x9cwet endxe2x80x9d of the papermaking machine. In another preferred embodiment, the micro-encapsulated grit is applied using a slot orifice coater positioned at the wet end of paper machine. In this preferred embodiment, the use of a slot orifice coater (as contrasted with a secondary headbox) increases the efficiency and uniformity of the micro-encapsulated grit application and reduces waste. In an alternate preferred embodiment, the micro-encapsulated grit is applied at the xe2x80x9cdry endxe2x80x9d of the papermaking machinery thereby preventing unnecessary wear on the paper drying machinery and felts. In this preferred embodiment, the micro-encapsulated grit is preferably only partially cured thereby enhancing the ability of the particles to adhere to the paper.
In the preferred embodiment where the micro-encapsulated grit is applied through the primary headbox, the preferably fully cured micro-encapsulated grit is mixed directly into the paper slurry prior to the deposition of the slurry on the paper wire. This method is preferable to other methods in that the micro-encapsulated grit is incorporated in the paper throughout the z-direction thereby enhancing the long-term abrasion resistant qualities of the resultant paper. The drawbacks of this method include some damage to the recycling pumps and slurry tank as well as potential loss of micro-encapsulated grit to the floor, etc. due to the relatively inefficient nature of the headbox application method.
In the preferred embodiment where the micro-encapsulated grit is applied using a secondary headbox, the secondary headbox can be located anywhere downstream of the primary headbox prior to the dryers, i.e. anywhere on the xe2x80x9cwet end.xe2x80x9d This method is preferable to other methods, such as dry-end addition, in that the micro-encapsulated grit is incorporated in the paper throughout the z-direction, and the extent to which the micro-encapsulated grit penetrates the paper fibers is adjustable depending on how far down the wire the secondary headbox is located. As mentioned above, the incorporation of the micro-encapsulated grit particles in the z-direction enhances the long-term abrasion resistant qualities of the paper made by the process. The drawbacks of this method, as in the use of the primary headbox, include some damage to the recycling pumps and slurry tanks as well as potential loss of micro-encapsulated grit to the floor, etc. due to the relatively inefficient nature of the headbox application method.
In the preferred embodiment where the micro-encapsulated grit is applied using a slot orifice coater, the slot orifice coating head applicator is may be positioned anywhere after the primary headbox and before the dryers, but it is preferably located near and, more preferably, immediately after the dry line, i.e., the point at which the deposited fibers begin to exhibit consolidation and there is no layer of surface water. Preferably, the slot orifice coater includes a bead-type or curtain-type applicator, and is most preferably a curtain-type applicator. Also it is preferred that the slot orifice coater is used in conjunction with a positive displacement pump which enables a predetermined amount of the micro-encapsulated grit composition to be evenly distributed across the surface of the cellulosic paper sheet. A static mixer may be incorporated in the slot orifice coater supply line to prevent or reduce the amount of micro-encapsulated grit settling out of the slurry.
In the preferred embodiment wherein the micro-encapsulated grit is deposited on the paper web at the dry end of the papermaking machine, i.e. after the dryers, the grit is applied to the web using a powder coater or similar-type process equipment to evenly distribute the micro-encapsulated grit on the web. In this preferred embodiment, the micro-encapsulated grit is preferably only partially cured thereby facilitating the adhesion of the grit to the paper web. Preferably, if the dryer cans have imparted too much heat to the web creating a xe2x80x9ctackyxe2x80x9d surface after the micro-encapsulated grit has been deposited, an extra set of chilled rollers is supplied to cure the xe2x80x9ctackyxe2x80x9d web prior to winding.
Other objects and advantages will be apparent from the following description, the drawings and the appended claims.